It is well known to prepare a suspension of platinum particles by reducing a platinum salt in an aqueous solution with hydrogen, for example. The platinum particles thus produced usually have an average size of at least 10 nm. The suspension, however, contains particles of various sizes, which is due to the fact that small particles have a tendency to unite to form bigger particles during the reduction. This is a disadvantage if the particles are to be used for catalytic purposes, because in this case small particles of approximately the same size are desired. It is another inconvenience that the suspension referred to is not quite stable. The particles have a tendency to settle.